Following the destruction of the Temple, there were remnants of Jews throughout Israel and waged a rebellion against the Roman Occupation. After many years of hostility and oppression, the Jews were led by a dynamic and heroic leader who managed to keep the mighty Roman Army at bay for nearly three years.
Bar Kochba organized a large guerilla army and succeeded in actually throwing the Romans out of Jerusalem and Israel and establishing, albeit for a very brief period, an independent Jewish state. The Talmud (Sanhedrin 97b) states that he established an independent kingdom that lasted for two and half years.
At the time, however, Bar Kochba ― who was a man of tremendous leadership abilities ― managed to unite the entire Jewish people around him. Bar Kochba initially was effective in uniting the factions and the Jews were a force to be reckoned with. They overran the Romans, threw them out of the land of Israel, declared independence and even minted coins. The Romans pushed back and began to supress the Jewish rebellion. Bar Kochba made his final stand in the city of Betar, which is to the southwest of Jerusalem. You can go visit it today, thought ancient Betar has not been excavated. The Talmud (in Gittin 57a) relates what happened in Betar:
They had the custom in Betar that when a baby boy was born they planted a cedar tree and for a baby girl they planted a pine tree, and when they would marry they would cut them down and make a marriage canopy of the branches. One day the daughter of Caesar was passing and the shaft of her litter broke. They cut down a cedar and brought it to her. The Jews of Betar fell upon them and beat them. They reported to Caesar that the Jews were rebelling and marched against them... they killed [Jewish] men, women and children until their blood flowed into the Mediterranean Sea... It was taught that for seven years the gentiles cultivated their vineyards with the blood of Israel without requiring manure for fertilization.
The city fell on the saddest day in the Jewish calendar ― the 9th of Av of the year 135, the same date as both the First and the Second Temple fell.
The Romans, in their fury, did not want to allow the Jewish bodies to be buried; they wanted to leave them out in the open to rot. According to tradition, the bodies lay in the open for months but did not rot. Today, when Jews say the Grace after Meals, Birkat HaMazon, they add a special blessing (ha tov u'mativ) as a way of thanking God for this act of mercy in Betar.
Many have wondered about the appropriateness of the blessing of Hatov U’mativ which literally means that G-d is good and bestows good. True, the bodies didn't compose after so many Jew were killed in the battlefield at the hands of the Romans. However, it was a gruesome defeat which was the final nail in the coffin that the Romans nailed in and pretty much wiped out the Jewish presence from the Land of Israel for many hundreds of years. Is this the time to start praising G-d for being good and bestowing kindness??
The Meshech Cochma provides a fascinating insight into this. After the defeat of Bar Kochba and the Jews in Beitar, the obituary on Klal Yisroel was being circulated throughout the world. After all, the mighty Roman Empire had destroyed the mighty Jewish Temple and displaced the Jews into exile. The last hurrah that was waged by Bar Kochba in Beitar was squashed and there was simply no way that Jews would have any future. It seemed that G-d had finally abandoned the Jews and we would be relegated into the dustbin of history. Sometime later when they noticed the bodies didn’t decompose after several months, they realized there was a larger message beyond this specific incident. It was a sign from G-d in arguably the darkest chapter of Jewish history, that despite the terrible defeat and destruction, the Jewish people would continue to endure and see brighter days ahead. That is the deeper message of this blessing of HaTov U’mativ.